I think that you folks are dramatically underestimating how many people there are in the world at this point.
There are seven billion people on this planet. Its a good bet that the zombie apocalypse only took care of about a quarter of that in the first year.
Sorry, but think you're using bad math. Of your 7 billion people, a little over 50% of them live in a city somewhere in the world. So, there's a little over 3.5 billion dead (or undead) within a week or so, give or take. That's just official urban areas, I'm guessing it's closer to 65-70% if you can count suburban areas, and anywhere within a few hours' walk of a city. Now we're down to around 2.1 billion give or take. Mostly in the first couple weeks. Might have some small pockets that hold out a while, but there's no food/water in a city that will keep things going for long, and no more coming. Plus, the aforementioned billions of nearby zombies, so you're mostly toast.
Probably the most unlikely part of Walking Dead is how close to Atlanta the survivor group is/was. Make a lot more sense if they came from the sticks, and were heading into Atlanta because of the CDC and whatnot. If they started there, seems unlikely they'd get out.
So anyway, down to around 30% of the world's population (give or take) within a few weeks. Now release the zombie hordes, cut off food supplies, allow for human/human violence to ensue, natural disasters, failure of a few nuclear power plants, and a winter to survive on their own, and can't imagine there's more than 10% or so of the population that has a chance, and they're in the sparsely-populated 3rd world countries where things like food deliveries and electricity weren't going to be missed.
I'm thinking 90% of the population is already zombies or dead. At least in the area they are.
There is nothing to support the idea that 90% of the population is dead. There just are not enough bodies/zombies around to account for a population that large.
Because it's bad tv (plus hard to film) if it's just a pile of dead bodies everywhere. Half a million people in Atlanta, goes to a couple million if you count nearby outlying areas. Obviously we didn't see a couple million zombies or corpses. We also saw about 20 survivors, so feel free to extend those numbers out as far as you like. If there's even 1000 zombies seen for every 20 survivors, you're still looking at .001% survival rate. (projects to 70k humans if you use worldwide numbers, but some areas would be much less impacted, and wouldn't see such bad numbers.)
Even if you assume that there's more compared to our 20 or so we've seen, I'll give you a nice big 500 (not anywhere near what wev'e seen total). Works out to .025%, 1.8 million in the world.
There aren't enough zombies PLUS humans PLUS dead bodies to account for the whole population anyway (or even a decent percent), so can't use that to begin with. Most appocolyptic shows/movies usually accept the conceit that someone magically cleaned up a lot of the bodies...